Dual-sided substrate measurement apparatus and methods

ABSTRACT

An apparatus for measuring the relative positions of frontside and backside alignment marks located on opposite sides of a substrate is disclosed. The apparatus includes upper and lower optical systems that allow for simultaneous imaging of frontside and backside alignment marks. The frontside and backside alignment mark images are processed to determine the relative position of the marks, as a measurement of the alignment and/or overlay performance of the tool that formed the marks on the substrate.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to alignment metrology, and in particularto apparatus and methods for measuring alignment and/or overlay accuracyof images formed in or on a substrate.

2. Background Information

In semiconductor manufacturing, the processing steps for fabricating asemiconductor device (e.g., an integrated circuit) include exposing asubstrate, such as a semiconductor wafer coated with photosensitivematerial, using a lithographic exposure apparatus. This exposureinvolves forming images at precise locations on the substrate. In oneexample, the substrate, which resides on a substrate stage, is alignedto a reticle residing on a reticle stage. The reticle includes a patternof a particular device layer, as well as alignment marks. The alignmentis performed using an alignment apparatus, such as that disclosed inU.S. Pat. No. 5,621,813, which patent is incorporated herein byreference.

After aligning the substrate, the reticle is illuminated with radiationand the reticle pattern is imaged in the photoresist. The exposedphotoresist is then developed and the substrate etched to transfer theresist pattern to the substrate.

The region of the substrate over which the reticle is imaged issometimes referred to an “exposure field”. This alignment and exposureprocess can be performed on a variety of lithography apparatus, such asstep and repeat, step-and-scan, contact and proximity apparatus.

Typically, the first device layer is aligned to a feature or mark on thesubstrate, such as a flat or notch and the substrate edge. Thesubsequent layers are then aligned relative to this first layer and/orto each other. “Alignment”, as the term is used herein, refers to theposition of the center of the exposure field relative to a referencelocation. “Overlay”, as the term is used herein, refers to the positionand orientation of the exposure field relative to an ideal exposurefield, and involves measuring the position of multiple points perexposure field. Thus for example, a particular exposure field can haveperfect alignment but poor overlay, e.g., if the exposure field isimaged with rotation or distortion. “Alignment mark”, as the term isused herein, refers to a feature on either side of the substrate whoseposition can be measured to establish the degree of either alignment oroverlay of a given exposure field.

To gauge the degree to which exposure fields are aligned and overlayedon the substrate, it is necessary to have the capability of makingalignment and overlay measurements. This is typically accomplished bymeasuring the position of an alignment mark in the exposure fieldrelative to another alignment mark or feature formed somewhere else onthe substrate. By measuring alignment or overlay for a given substrateor set of substrates, adjustments can be made to the exposure tool toreduce or eliminate tool-related sources of alignment and/or overlayerrors. Reducing such errors increases the likelihood that the devicebeing manufactured by the exposure tool will perform to itsspecification.

Certain lithographic exposure apparatus perform exposures of thefrontside of the substrate while aligning to features (e.g., alignmentmarks) on the backside of the substrate. Conventional apparatus andtechniques for quantifying frontside to backside alignment performanceinvolve measuring two marks at two widely separated points on thefrontside of the substrate in relation to two corresponding backsidemarks. This typically involves viewing the alignment marks on thebackside through the frontside of the substrate using, e.g., a singleoptical system operating with infrared light. However, this measurementtechnique cannot accurately measure mask (reticle) run-out orstepping-grid errors. Thus, the actual alignment performance over theentire substrate is not always properly characterized by conventionalapparatus and techniques. Further, not all substrates are transparentenough to allow for both sides to be viewed by a single optical systempositioned on one side of the substrate.

What is needed therefore are alignment apparatus and method forproviding a more accurate characterization of alignment and overlayperformance of an exposure tool.

SUMMARY OF THE INVENTION

A first aspect of the invention is an apparatus that includes a stageassembly capable of movably supporting a substrate. The substrate has afrontside with frontside alignment marks and a backside with backsidealignment marks. An upper optical system is movably arranged above thestage assembly and the frontside of the substrate so as to be in opticalcommunication with the frontside of the substrate to form an image ofone or more of the frontside alignment marks. A lower optical system isarranged relative to the upper optical system and beneath the stageassembly so as to be in optical communication with the backside of thesubstrate to form an image of one or more of the backside alignmentmarks. The images of the frontside and backside alignment marks arecaptured by a frame grabber coupled to the upper and lower opticalsystems. The images are then processed (e.g., in a central processingunit connected to the frame grabber) to determine the relative positions(i.e., the alignment) of the alignment marks, and hence the quality ofthe alignment and/or overlay performance of the tool that formed thealignment marks on the substrate.

A second aspect of the invention is a method of measuring a substratehaving a frontside with frontside alignment marks and a backside withbackside alignment marks. The method includes capturing a first image ofa select frontside alignment mark with a an upper optical systemarranged adjacent the frontside of the substrate. The method alsoincludes capturing a second image of a select backside alignment markwith a lower optical system arranged adjacent the backside of thesubstrate and aligned relative to the upper optical system. The methodalso includes processing the first and second images to determine alateral offset between the select frontside and backside alignmentmarks.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of an embodiment of the dual-sidedalignment metrology apparatus of the present invention;

FIG. 2 is a side view of a substrate having frontside and backsidealignment marks;

FIG. 3 is a close-up side view of an embodiment of the stage assembly ofthe apparatus of FIG. 1;

FIG. 4 is a plan view of an embodiment of the calibration fiducial usedto calibrate the apparatus of FIG. 1;

FIG. 5A is an example of an alignment mark in the form of a solid cross;

FIG. 5B is an example of an alignment mark in the form of hollow cross;

FIG. 5C is an example of superimposed images of the solid crossalignment mark of FIG. 5A and the hollow cross alignment mark of FIG. 5Bas might appear on the display of the apparatus of FIG. 1;

FIG. 6A is an example of an alignment mark in the form of a solid box;

FIG. 6B is an example of an alignment mark in the form of hollow box;

FIG. 6C is an example of superimposed images of the solid box alignmentmark of FIG. 6A and the hollow box alignment mark of FIG. 6B, as mightappear on the display of the apparatus of FIG. 1;

FIG. 7 is a cross-sectional view of an example of a linear Z-axisalignment tool;

FIG. 8 shows images of frontside and backside alignment marks as mightappear in two separate windows on a display of the apparatus of FIG. 1;and

FIG. 9 is an example embodiment of a table-top measurement apparatusincorporating the dual-sided alignment metrology apparatus of FIG. 1.

DETAILED DESCRIPTION OF THE INVENTION

In the following detailed description of the embodiments of theinvention, reference is made to the accompanying drawings that form apart hereof, and in which is shown by way of illustration specificembodiments in which the invention may be practiced. These embodimentsare described in sufficient detail to enable those skilled in the art topractice the invention, and it is to be understood that otherembodiments may be utilized and that changes may be made withoutdeparting from the scope of the present invention. The followingdetailed description is, therefore, not to be taken in a limiting sense,and the scope of the present invention is defined only by the appendedclaims.

FIG. 1 is a schematic diagram of an example embodiment of a dual-sidedmeasurement apparatus 10 according to the present invention. Acoordinate system 12 is shown for reference. Apparatus 10 is referred toas “a measurement apparatus” for the sake of simplicity. It will beunderstood by those skilled in the art that the apparatus is capable ofperforming alignment and overlay measurements as described below.

An overview of apparatus 10 and its main components is first provided.Then each of the main apparatus components is discussed in greaterdetail, along with methods of using the apparatus for measuring asubstrate.

Apparatus Overview

Alignment apparatus 10 has a stage assembly 18 that includes a substratestage 20 with an upper surface 22 and a lower surface 24. In an exampleembodiment, substrate stage 20 is capable of moving in at least theX-direction, the Y-direction and the θ_(Z) direction. Stage assembly 18further includes a platen 40 (also referred to as a “chuck”) arrangedabove (e.g., atop) substrate stage upper surface 22. Platen 40 has aperiphery 41, an upper surface 42 and a lower surface 44. Platen uppersurface 42 is capable of supporting a substrate 50 having a frontside 52and a backside 54. In an example embodiment, platen 40 is transparent toselect wavelengths of radiation so that backside 54 can be viewedthrough the platen.

FIG. 2 is a side view of an example substrate 50. Substrate 50 includesfront-side alignment marks 60 on frontside 52 and corresponding backsidealignment marks 62 (not shown in FIG. 1) on backside 54. In an exampleembodiment, frontside and backside alignment marks have a size on thescale of microns or tens of microns.

With reference again to FIG. 1, residing above (i.e., adjacent)substrate frontside 52 (or platen upper surface 42 when the substrate isabsent) is an upper optical system 66 with an axis A1. Residing beneath(i.e., adjacent) substrate backside 54 (or platen lower surface 44 whenthe substrate is absent) is a lower optical system 76 with an axis A2.In an example embodiment, a controller 80 controls the operation ofapparatus 10 and/or processing information therefrom coupled to at leastone of upper optical system 66, lower optical system 76, and stageassembly 18. Further in an example embodiment, a display 84 and an inputdevice 86 are coupled to controller 80.

Stage Assembly

FIG. 3 is a detailed side view of stage assembly 18. The variouscomponents of stage assembly 18 are discussed separately below.

Translation Stages and Platen

In an example embodiment, stage assembly 18 includes a rotation(“theta”) stage 102 upon which is mounted an X-translation stage 104 anda Y-translation stage 106. It is easier to rotate the image if rotationstage 102 is tied to ground and its alignment axis is aligned to themicroscope axes A1 and A2. With this arrangement, the rotation ofsubstrate 50 is always centered in the field of view of the upper orlower optical system no matter what part of the substrate is beingviewed. In practice, stages 102, 104 and 106 can be arranged in anyorder along the Z-direction. In an alternate example embodiment, therotation stage can be eliminated and the substrate can be manuallyrotated on the platen.

In an example embodiment, X- and Y-translation stages 104 and 106 eachinclude a coarse linear position control knob 110 and a fine positioncontrol knob 112. Further, rotation stage 102 includes coarse and fineangular position control knobs 114 and 116. Control knobs 110 and 112are coupled to respective drive mechanisms 120 that drive the movementof the X- and Y-translation stages in response to movement is of thecontrol knobs. Likewise, control knobs 114 and 116 are coupled to adrive mechanism 122 that drives the angular movement of rotation stage102.

In an alternate example, the control knobs for the rotational drivemechanism 122 can be eliminated and rotation can be achieved by manuallyrotating rotation stage 102 between marked positions or hard stops 180degrees apart.

In an example embodiment, coarse linear position control knobs 110 arecapable of tenth millimeter resolution (e.g., 25 mm per revolution ofthe knob), and fine position control knobs 112 are capable of 0.1 micronresolution. Further in an example embodiment, angular control knobs 114and 116 are capable of producing a 180 degree rotation within a ½ degreeangular resolution.

In an example embodiment, one or more of control knobs 110, 112, 114 and116 are manually operable to adjust stage assembly 18 to control themovement and position of platen 40 and substrate 50 residing thereon.

In another example embodiment, drive mechanisms 120 and 122 are coupledto controller 80 via signal lines 126, to automatically adjust stageassembly 18 to control the movement and position of platen 40 andsubstrate 50 residing thereon. This is accomplished by controller 80providing a drive signal 128 over at least one of lines 126. In exampleembodiments, lines 126 and the other lines connected to controller 80 asdescribed below are either electrical wires or optical fibers, and thesignals transmitted over the lines are either electrical or opticalsignals.

In an example embodiment, substrate stage 20 of stage assembly 18 has arange of motion that allows upper optical system 66 to opticallycommunicate with any point on frontside 52 of substrate 50, and thatallows lower optical system 76 to optically communicate with any pointon backside 54.

Further in an example embodiment, substrate stage 20 is capable ofproviding 180° rotation about any point on substrate frontside 52 orsubstrate backside 54. An advantage of such capability is that analignment mark can be imaged and rotated without the image leaving thefield of view of the particular optical system. The ability to image andmeasure an alignment mark at different orientations allows for themeasurement and reduction or elimination of orientation-dependenterrors. This in turn provides apparatus 10 with improved measurementcapability.

Stage assembly 18 must permit backside alignment marks 62 to be viewedby lower optical system 76. Accordingly, in one example embodimentsubstrate stage 20 includes an aperture 127 (the inner walls of whichare illustrated by dashed lines 128 in FIG. 3) through which backside 54of substrate 50 can be viewed by lower optical system 76.

Likewise, platen 40 residing atop substrate stage 20 must permit loweroptical system 76 to view backside alignment marks 62 on substrate 50.Thus, in an example embodiment, rather than having an aperture formedtherein, platen 40 is transparent to provide a clear view of substratebackside 54 by lower optical system 76, while also providing support forsubstrate 50. In an example embodiment, platen 40 is made from opticallyflat glass to minimize imaging errors (e.g., distortion) when imagingbackside alignment marks 62.

Calibration Fiducial

In an example embodiment, apparatus 10 is calibrated to ensure thehighest level of measurement accuracy. FIG. 4 is a plan view of acalibration fiducial 136 used in an example embodiment to calibrateapparatus 10, as described in greater detail below.

Calibration fiducial 136 includes an array 138 of reference marks 140formed on a substrate 142. In an example embodiment, reference marks 140are preferably arranged over substrate 142 to cover 80% or greater ofthe fields of view of the upper and lower optical systems 66 and 76. Inexample embodiments, reference marks 140 are opaque or semi-opaque.Further in an example embodiment, reference marks 140 are formed from ametal such as chrome, and substrate 142 is made from a transparent orsemi-transparent material, such as glass (e.g., fused silica).

In one example embodiment, calibration fiducial 136 is included inplaten 40, in the plane of surface 42 as illustrated in FIG. 3. Thecalibration fiducial is arranged such that reference marks 140 can beimaged by both upper and lower optical systems 66 and 76. Alternativelythe calibration fiducial can be constructed on a separate glass issubstrate and placed on the platen for calibration.

In an additional example embodiment, reference marks 140 are located onboth sides of substrate 142 but in this case the top and bottom targetshave to be in near perfect alignment and the calibration fiducial mustbe constructed on a separate substrate and placed on the platen.

Alignment Pins

With reference again to FIG. 3, in an example embodiment stage assembly18 further includes alignment pins 160 that are extendable through uppersurface 42 of platen 40 to assist in positioning substrate 50 in X, Yand rotation on the upper surface of the platen. Alignment pins 160 arecoupled to a pin driver 162 via a coupling conduit 164 (only one conduitis shown). Pin driver 162 is coupled via line 166 to controller 80,which controls the extension and retraction of pins 160. In theretracted state, pins 160 are flush with or lie below platen uppersurface 42. In an example embodiment, pin driver 162 is an air or vacuumpump and coupling conduit 164 is an air or vacuum line. In an alternateexample embodiment, the pins can be manually placed or removed atdifferent locations to adapt to different substrate sizes and types, andthey need not be retractable if they are arranged to clear upper opticalsystem 66.

Vacuum Substrate Securing System

With continuing reference to FIG. 3, stage assembly 18 also includes avacuum substrate securing system 180 for securing substrate 50 to platen40. System 180 includes a vacuum pump 186 and vacuum lines 188 (only onevacuum line is shown). Vacuum line 188 is connected to vacuum pump 186and to vacuum holes 192 in platen upper surface 42. In an exampleembodiment, vacuum holes 192 are formed near platen periphery 41.Substrate 50 is thus secured to platen 40 by a vacuum provided throughholes 192. Securing substrate 50 to platen 40 is desirable to preventunwanted movement of the substrate during the measurement process. In anexample embodiment, vacuum pump 186 is coupled via line 194 tocontroller 80, which controls the operation of system 180.

Upper Optical System

With reference again to FIG. 1, upper optical system 66 includes amicroscope objective 200. Microscope objective 200 is optically coupledto a detector 204, which is is coupled to controller 80 via line 206. Inan example embodiment, microscope objective and detector 200 constitutea video microscope 210.

In an example embodiment, detector 204 is a CCD array. Further in anexample embodiment, CCD array is black and white. An example resolutionof detector 204 is 512×408 pixels. Upper optical system 66 has a fieldviewing area FVA1 on the substrate and a corresponding detector viewingarea DVA1 on the detector. In addition, microscope objective 200 has anumerical aperture NA1, a depth of field DOF1, a working distance WD1, amagnification M1, and an operating (viewing or imaging) wavelength λ1.

Table 1 below lists values for the above parameters for an exampleembodiment of upper optical system 66.

TABLE 1 Example parameter values for embodiment of upper optical systemPARAMETER VALUE NA1 0.28 DOF1 +/−6.4 microns WD1 33.5 mm λ1 0.5-1.0microns M1 ~20X DVA1 5 mm × 5 mm FVA1 250 × 250 microns

In an example embodiment, microscope objective 200 is manually movablein the Z-direction via a focusing knob 214 coupled to a focusingmechanism 216 to bring upper optical system 66 into focus relative tosubstrate 50. In an example embodiment, focusing mechanism 216 iscoupled directly to controller 80 via line 218 to provide for automaticfocus control of upper optical system 66.

Also in an example embodiment, upper optical system 66 includes anillumination apparatus 240 comprising, for example, a light source 244optically coupled to an optical system 246. In an example embodiment,light source 244 is a light-emitting diode. In an example embodiment,optical system 246 includes an optical fiber. Further in an exampleembodiment, optical system 246 includes a beamsplitter 248. Beamsplitter248 is arranged along axis A1 in the optical path between substrate isfrontside 52 and detector 204. In an example embodiment, beamsplitter248 is arranged between microscope objective 200 and detector 204, asshown in FIG. 1.

In an example embodiment, the wavelength λ1 emitted by light source 244is in the range between 500 and 1000 nm. Further in an exampleembodiment, light source 244 is coupled to controller 80 via line 250for automated control of the illumination of substrate frontside 52.

In an example embodiment, upper optical system 66 is coupled to a stage260 capable of moving the upper optical system in at least one of the X,Y, Z, θ_(X), θ_(Y) and θ_(Z) directions to position the upper opticalsystem relative to stage assembly 18. Movement in the Z-direction allowsupper optical system 66 to focus onto one or more of front-sidealignment marks 60. Stage 260 may be moved manually in the X, Y, Zθ_(X), θ_(Y) and θ_(Z) directions via respective control knobs 268.

In an example embodiment, control knobs 268 have respective resolutionson the order of a tenth of a micron and 10 microradians. In an exampleembodiment, stage 260 is coupled to controller 80 via line 272 toprovide for automatic control of the movement and positioning of upperoptical system 66. In an alternate example embodiment, only the Z axiswould have adjustment knobs, and a one-time alignment adjustment of theother degrees of freedom would be done with external tooling.

Lower Optical System

In an example embodiment, lower optical system 76 is the same as, orsubstantially the same as, upper optical system 66, described above.

With continuing reference to FIG. 1, lower optical system 76 includes amicroscope objective 300 optically coupled to a detector 304, which iscoupled to controller 80 via line 306. In an example embodiment,microscope objective 300 and detector 304 constitute a video microscope310.

In an example embodiment, detector 304 is a CCD array. Further in anexample embodiment, CCD array is black and white. An example resolutionof detector 304 is 512×480 pixels. Lower optical system 76 has a fieldviewing area FVA2 on the substrate and a corresponding detector viewingarea DVA2 on the detector. Microscope objective 300 has a numericalaperture NA2, a depth of field DOF2, a working distance WD2, amagnification M2, and an operating (viewing or imaging) wavelength λ2.In an example embodiment, lower optical system 76 has the same orsubstantially the same parameter values as those set forth above inTable 1 for upper optical system 66.

In an example embodiment, microscope objective 300 is movable in theZ-direction via a focusing knob 314 coupled to a focusing mechanism 316to focus lower optical system 76. In an example embodiment, focusingmechanism 316 is coupled directly to controller 80 via line 318 toprovide for automatic focus control of lower optical system 76.

Also in an example embodiment, lower optical system 76 includes anillumination apparatus 340 comprising, for example, a light source 344optically coupled via an optical system 346. In an example embodiment,light source 344 is a light-emitting diode. In an example embodiment,optical system 346 includes an optical fiber. Further in an exampleembodiment, optical system 346 includes a beamsplitter 348. Beamsplitter348 is arranged between microscope objective 300 and detector 304, asshown in FIG. 1. In an example embodiment, the wavelength λ emitted bylight source 344 is in the range between 500 and 1000 nm. Light source344 is coupled to controller 80 via line 350 to control the illuminationof substrate backside 54.

In an example embodiment, lower optical system 76 is coupled to a stage360 capable of moving the lower optical system in at least one of the X,Y, Z, θ_(X), θ_(Y) and θ_(Z) directions to position the lower opticalsystem relative to stage assembly 18. Movement in the Z-direction allowslower optical system 76 to focus onto one or more of backside alignmentmarks 62. Stage 360 may be moved manually in the X, Y, Z θ_(X), Ξ_(Y)and θ_(Z) directions via respective control knobs 368.

In an example embodiment, control knobs 368 have respective resolutionson the order of a tenth of a micron and 10 microradians. In an exampleembodiment, stage 360 is coupled to controller 80 via line 372 toprovide for automatic control of the movement and positioning of loweroptical system 66.

In an alternate example embodiment, only the Z axis would haveadjustment knobs, and a one-time alignment adjustment of the otherdegrees of freedom would be done with external tooling (not shown).

In another example embodiment, stage 360 simply serves as an immobilesupport that holds lower optical system fixed in space relative to stageassembly 18. In this example embodiment, the movement of stage assembly18 in the Z-direction is used to provide focusing capability for loweroptical system 76 to image backside alignment marks 62.

In an alternate example embodiment, the Z-direction adjustment iseliminated and the substrate backside surface is made to remain withinthe depth-of-focus of lower microscope 300 over the full travel of thestage.

Frontside and Backside Alignment Marks

With reference now to FIGS. 5A-5C and 6A-6C, there are shown twodifferent example embodiments of frontside and backside alignment marks60 and 62. In an example embodiment, frontside and backside alignmentmarks 60 and 62 are formed using standard photolithographic processes.

Further in an example embodiment, frontside alignment marks 60 areformed first on substrate frontside 52 of substrate 50 using standardphotolithographic tools (not shown), and then the backside alignmentmarks 62 are formed on substrate backside 54 using the samephotolithographic exposure tool. Almost any shape or type of alignmentmark can be used, as long as the center of the mark is able to bedetermined. The alignment marks discussed immediately below are examplesthat differ from the frontside to the backside. Alternatively, thefrontside and backside alignment marks can be the same shape and/orsize.

Thus, in one example embodiment, frontside alignment mark 60 is a solidcross 402 (FIG. 5A) with dimensions L1 and L2, while backside alignmentmark 62 is a hollow cross 404 (FIG. 5B) with corresponding dimensions L3and L4, which are slightly larger than dimensions L1 and L2. FIG. 5Cillustrates the overlay of an image 402′ of solid cross 402 from upperoptical system 66 with an image 404′ of hollow cross 404 from loweroptical system 76, as might be displayed on display 84. In an exampleembodiment, cross-hairs 410 are optionally used to assist in aligningand determining the relative position of the two alignment marks.

In an example embodiment, dimension L1 is about 10 microns, anddimension L2 is about 2 microns. The alignment mark geometry illustratedin FIG. 5C is sometimes referred to as “cross in a cross”. Generally,the polarity of alignment marks 60 and 62 can be either positive ornegative. Further, alignment marks 402 and 404 can be used on eitherside of the substrate.

In another example embodiment, frontside alignment mark 60 is a solidbox 420 (FIG. 6A), while backside alignment mark 62 is a hollow box 422(FIG. 5B) with dimensions somewhat larger than that of solid box 420.FIG. 6C illustrates the overlay of an image 420′ of solid box 420 fromupper optical system 66 with an image 422′ of hollow box 422 from loweroptical system 76 that might be displayed on display 84. In an exampleembodiment, cross-hairs 410 are optionally used to assist in aligningand determining the relative position of the two alignment marks.

In an example embodiment, solid box 420 and hollow box 422 are square.Further in the example embodiment, solid box 422 has a dimension L5 ofabout 10 microns and hollow box 592 has an inside dimension L6 of about21 microns and an outside dimension L7 of about 23 microns. Thisalignment mark geometry is sometimes referred to as “box in a box”. Thepolarity of alignment marks 60 and 62 can be either positive ornegative, and the alignment marks can be used on either side of thesubstrate.

Controller and Software

With reference again to FIG. 1, in an example embodiment controller 80includes a frame grabber 500 for capturing images from detectors 204 and304. An example frame grabber 500 is a COGNEX™ 8120 frame grabbing boardavailable from Cognex Corporation, Natick, Mass. Controller 80 furtherincludes a microprocessor 502 and a memory unit 504, which is connectedto the microprocessor by a bus 510. Memory unit 504 includes the abilityto retrievably store data. In an example embodiment, memory unit 504 iscapable of storing past alignment and/or overlay measurements, videoimages of alignment marks 60 and 62 (e.g., as bitmap files), and thelike.

Controller 80 also includes a drive unit 512, such as a compact disk(CD) drive or a floppy drive, adapted to receive, read from and write toa computer-readable medium 514, such as a CD or a floppy disk.

Microprocessor 502 includes image-processing software that processesimages of frontside and backside alignment marks 60 and 62 captured byframe grabber 500. In an example embodiment, frame grabber 500 andmicroprocessor 502 constitute a frame-grabbing board. The imageprocessing software includes, in an example embodiment, one or more ofthe following features: Pattern training, target capture, targetposition measurement, video camera pixel calibration (x-scale, y-scale,x rotation, y rotation), top-to-bottom offset calibration, Z-travelcalibration, top-to-bottom misregistration measurement, ability to logmultiple measurements and/or images to memory, and ability to performbasic statistical calculations on multiple measurements. Example imageprocessing software is known by the trademark PATMAX, which is availablewith the COGNEX 8120 frame grabber from Cognex Corporation.

Microprocessor 502 also includes software that embodies instructions forcarrying out the methods for controlling the operation of apparatus 10.Such methods are described below.

In an example embodiment, controller 80 is a personal computer (PC)running an operating system, such as WINDOWS NT or WINDOWS 2000.

Display and Input Device

With continuing reference to FIG. 1, display 84 is used to displayimages of the front-side alignment marks 60 and backside alignment marks62 as captured by upper and lower optical systems 66 and 76,respectively. In an example embodiment, display is part of a cathode-raytube (CRT) or a liquid crystal device (LCD) flat-panel.

Input device 86 is used to provide input information into controller 80to operate and control apparatus 10. In an example embodiment, inputdevice 86 includes a keyboard and/or a mouse. Input device 86 may alsobe included as part of display 84, e.g., as a touch-activated screen.

Methods of Calibration

As mentioned above, in an example embodiment, apparatus 10 is calibratedto obtain the most accurate and precise measurements of alignment and/oroverlay. Calibration eliminates measurement errors, includingtool-induced shift (TIS). The calibration includes ensuring that: (1)the linear Z-direction of travel of the upper and lower optical systemsis true; (2) the upper and lower optical systems 66 and 76 are alignedrelative to one another, i.e., that axes A1 and A2 are coaxial along theZ-direction and that the X and Y axes are aligned in rotation and thetransverse measurement scale matches a known reference; and (3) thatsystematic residual errors from other system components are measured.The first calibration is referred to herein as “Linear Z-axisalignment”, the second calibration is referred to herein as “opticalsystem calibration” and the third calibration is referred to herein as“Tool Induced Shift” or “TIS”. These three calibrations are nowdiscussed in greater detail.

Linear Z-Axis Alignment

FIG. 7 is a cross-sectional view of a linear Z-axis alignment tool 600(hereinafter, “tool 600”). Tool 600 includes a substrate 602 having anupper surface 604 with upper surface targets 606, and a lower surface608 with lower surface targets 610. In one example embodiment, uppersurface targets 606 are the same as lower surface targets 610. Upper andlower surface targets 606 and 610 are aligned in the X and Y directions.

In an example embodiment, tool 600 is formed from two separate clearsubstrates (e.g., glass masks) that are first aligned with the upper andlower surface targets having the same (X,Y) coordinates in a planeparallel to lower surface 608, and then bonded together using, forexample, optical cement.

With reference also to FIG. 1, tool 600 is placed on platen 40. In theembodiment where lower optical system 76 is fixed in space, upperoptical system 66 is adjusted to image one of lower surface targets 610.The lower surface target image is then captured by frame grabber 500 andstored in memory unit 504. Upper optical system 66 is then moved in theZ-direction to image the corresponding upper surface target 606. Theupper surface target image is then captured by frame grabber 500 andstored in memory unit 504. Tool 600 is then rotated 180° about axis A2and the process is repeated.

Because upper and lower surface targets 606 and 610 have the same X andY positions, any measured offset in X and Y between the target images isdue to an X,Y deviation in the movement of the upper optical system asit travels in the Z-direction. The upper optical system can then beadjusted so that it travels directly along the Z-direction.

In the embodiment where lower optical system 76 is movable along theZ-direction, the above procedure is repeated using the lower opticalsystem.

Optical System Calibration

Optical system calibration involves ensuring that axes A1 and A2 arecoaxial. When performing a calibration using calibration fiducial 136(FIG. 4), upper optical system 66 and lower optical system 76 arepositioned so that each can view reference mark array 138 (FIG. 4).Then, reference mark array 138 is imaged with upper and lower opticalsystems 66 and 76, and the respective images captured. In oneembodiment, the image processing software processes the two images andcompares the locations of each of the reference marks to a referenceposition. Then one of the optical systems is moved (either manually orautomatically) to compensate for any offset. Manual alignment of axes A1and A2 can be performed by viewing the relative positions of the imagedreference marks on display 84 and moving one image with respect toanother until they are overlayed. Once the position that eliminates orminimizes the offset is found, the upper optical system, say, is fixed.At this point, upper optical system 66 is allowed to move only in theZ-direction for focusing.

Besides serving to align the two optical systems, imaging the referencemark array 138 with the lower and upper optical systems is used tocalibrate the co-ordinate system of each optical system so that thex-scale, y-scale, x rotation, and y rotation of detectors 204 and 304 isknown accurately.

Tool Induced Shift

Once the linear Z-axis and optical systems calibrations are performed,the Tool Induced Shift (TIS) can be measured and stored.

With reference also to FIG. 3, calibration fiducial 136 is used for thiscalibration. One of the reference marks 140 is then imaged by loweroptical system 76, captured by frame grabber 500 and its target positionis stored in memory unit 504. The same reference mark 140 is then imagedby upper optical system 66 and captured by frame grabber 500 and itstarget position is stored in memory unit 504. Calibration fiducial 136is then rotated 180° about axis A2 and the process is repeated.

Residual misalignment between corresponding lower and upper opticalsystems 76 and 66 can be estimated by subtracting the measurements takenat 0° and 180° and dividing by two. The misalignment is a function ofresidual errors from pixel calibration and linear Z-axis calibration.

Also included in this value will be XY errors due to stage tip and tilt,indicating that there may be dependency between the ITS value and theposition of stage assembly 18. Therefore to obtain the highest accuracyit is necessary to perform a ITS calibration over an array of stagepositions.

This information, stored in memory unit 504 can be used to automaticallycancel residual systematic error for all subsequent measurements,eliminating the need to perform a TIS measurement on a field-by-fieldbasis.

Measurement Methods

Once apparatus 10 is calibrated, then a particular substrate 50 can bemeasured. In an example embodiment, measurements are performed on afield-by-field basis, rather than relying on a limited number ofmeasurements at widely separated points on the substrate, as is commonin the prior art. Thus, in example embodiments of the measurementmethods of the present invention, tens to hundreds offrontside-to-backside alignment measurements are taken per substrate.Measurements are taken at different points on the substrate by movingstage assembly 18 so that select substrate locations are sequentiallyarranged between upper and lower optical systems 66 and 76.

In an example embodiment, each measurement compares the position of oneor more frontside alignment mark 60 to one or more correspondingbackside alignment marks 62. Further in the example embodiment, theposition (X1, Y1) of one frontside alignment mark 60 is compared to theposition (X2, Y2) of a corresponding backside alignment mark 62.

FIG. 8 shows images 60′ and 62′ of frontside and backside alignmentmarks 60 and 62 as might appear in two separate windows 656 and 658 ondisplay 84. In an example embodiment, the X,Y location (X1, Y1) for aselect frontside alignment mark 60 relative to an origin O (i.e.,reference point (X1=0, Y1=0) is determined by capturing image 60′ of afrontside alignment mark 60 using the image processing software inmicroprocessor 502 of controller 80.

Likewise, the X,Y location (X2, Y2) for a select backside alignment mark62 relative to the same common origin O is determined by capturing animage 62′ of the backside alignment mark and establishing its location(X2, Y2) relative to origin O using the image processing software. Theoffset between the marks relative to common origin O is then determinedby a straightforward calculation, e.g., ΔX=(X1−X2) and ΔY=(Y1−Y2). In anexample embodiment, the calculation is performed in microprocessor 502.

In another example embodiment, image 62′ of a backside alignment mark 62is centered at a point C2 in the field of view 662 of lower opticalsystem 76 by moving stage assembly 18. Then, an offset D1 betweenfrontside alignment mark 60 and center C1 of the field of view 666 ofupper optical system 66 (as indicated by cross-hairs 410) isestablished. This may be accomplished, for example, by automatically ormanually moving stage assembly 18 so that frontside alignment mark image60′ is centered in field of view 666 of the upper optical system, andrecording the displacement D1 from the original position (X1, Y1).

In another example embodiment, respective images 60′ and 62′ aredisplayed together on display 84 in the same window, as illustrated inFIGS. 5C and 6C, so that the offset is determined by the amount ofmovement needed to overlap images 60′ and 62′.

In an example embodiment, the alignment measurement data for each set offrontside and backside alignment marks is stored in memory unit 504.Once the measurement data for the desired number of sites has beencollected, the data is processed by controller 80 (e.g., bymicroprocessor 502 therein). The data processing includes, for example,listing the displacement values (ΔX, ΔY) for each site, and calculatingthe average and the standard deviation for displacement values.

An advantage of performing more than just a few measurements persubstrate is that enough data can be taken to ascertain the root causesof any alignment and/or overlay errors, such as from mask run-out,coating non-uniformities, substrate non-flatness, and grid errors. Theability to characterize and quantify alignment and/or overlay isimportant in modern semiconductor manufacturing because it allows one toredress the source of the errors to improve tool performance, andimproved tool performance generally leads to improved semiconductordevice performance.

Table-Top Apparatus Embodiment

FIG. 9 is side view of a table-top apparatus 700 that includesdual-sided alignment metrology apparatus 10 of FIG. 1. System 700includes a support frame 710 having a horizontal support member 714 withan upper surface 716 and a lower surface 718. Horizontal support member714 is supported above a surface 720 of a table 722 by legs 724connected to the horizontal support member, e.g., at lower surface 718.Table 722 is, in an example embodiment, an isolation table of the kindtypically used for high-magnification microscopes. Stage assembly 18 issupported by horizontal support member 714. In example embodiments,stage assembly 18 is supported on or within upper surface 716.

Apparatus 700 includes a support pedestal 740. In an example embodiment,support pedestal 740 has a base 746, a vertical columnar portion 750extending upward from the base, and a support arm 756 extendinghorizontally from the vertical column. Support arm 756 has an end 758 towhich upper optical system 66 is movably attached.

Base 746 of support pedestal 740 is supported by upper surface 716 ofhorizontal support member 714 such that upper optical system 66 issupported over stage assembly 18. Lower optical system 76 is attached tohorizontal support member 714 beneath stage assembly 18 so as to be inoptical communication with stage assembly 18, and in particular to asubstrate 50 when placed thereon.

In an example embodiment, upper and lower optical systems 66 and 76, aswell as stage assembly 18, are coupled to controller 80. In an exampleembodiment, display 84 and input device 86 are located close toapparatus 700 (not shown) so that a person operating apparatus 700 canview images of frontside and backside alignment marks 60 and 62 shown onthe display, as well as any other information relating to themeasurement of substrate 50.

The various elements depicted in the drawings are merelyrepresentational and are not drawn to scale. Certain proportions thereofmay be exaggerated, while others may be minimized. The drawings areintended to illustrate various implementations of the invention, whichcan be understood and appropriately carried out by those of ordinaryskill in the art.

While certain elements have been described herein relative to “upper”and “lower”, “horizontal” and “vertical”, and “above” and “below”, itwill be understood that these descriptors are relative, and that theycould be reversed if the elements were inverted, rotated, or mirrored.Therefore, these terms are not intended to be limiting.

In the foregoing Detailed Description, various features of the inventionare grouped together in various example embodiments for the purpose ofstreamlining the disclosure. This method of disclosure is not to beinterpreted as reflecting an intention that the claimed embodiments ofthe invention require more features than are expressly recited in eachclaim. Rather, as the following claims reflect, inventive subject matterlies in less than all features of a single disclosed embodiment. Thusthe following claims are hereby incorporated into the DetailedDescription, with each claim standing on its own as a separate preferredembodiment.

The many features and advantages of the present invention are apparentfrom the detailed specification, and, thus, it is intended by theappended claims to cover all such features and advantages of thedescribed apparatus that follow the true spirit and scope of theinvention. Furthermore, since numerous modifications and changes willreadily occur to those of skill in the art, it is not desired to limitthe invention to the exact construction and operation described herein.Accordingly, other embodiments are within the scope of the appendedclaims.

1. A method, comprising: positioning a substrate having a frontside withfrontside alignment marks and a backside with backside alignment marksbetween upper and a lower optical systems in optical communication withthe frontside and backside, respectively; capturing first and secondimages of a select frontside alignment mark and a select backsidealignment mark; and processing the first and second images to determinean offset between the select frontside alignment mark and the selectbackside alignment mark.
 2. The method of claim 1, wherein theprocessing includes processing the captured images to determinerespective centers of each of the select frontside and backsidealignment marks.
 3. The method of claim 1, wherein prior to positioningthe substrate, further including the step of capturing first and secondimages of reference marks on a calibration fiducial to align the firstand second optical systems and measure a residual systematic offset. 4.The method of claim 1, wherein prior to positioning the substrate,further including capturing first and second images of reference markson a calibration fiducial to measure the magnification and rotation ofthe first and second optical systems to determine a measurement scaleand relative rotation.
 5. The method of claim 1, wherein prior topositioning the substrate, further including capturing first and secondimages of reference marks on a Z axis alignment tool to measure a changein X and Y offset of the first optical system relative to the secondoptical system over a range of focus of the first optical system.
 6. Themethod of claim 1, including storing measurements of residual systematicoffsets as a function of stage X and Y position, offsets as a functionof Z focus position, and scale and rotation of the optical systems, anddetermining an absolute displacement between each of the selectfrontside and backside alignment marks.
 7. The method of claim 1,including performing the acts recited therein for any select pairs offrontside and backside alignment marks at any location on the substratewithout rotating the substrate to measure tool induced shift (TIS). 8.The method of claim 1, including displaying at least one of the firstand second images on a display.
 9. The method of claim 1, includingperforming the acts recited therein for two or more pairs of frontsideand backside alignment marks.
 10. The method of claim 1, includingperforming statistical analysis on any number of displacementmeasurements on select frontside and backside alignment marks anddisplaying the results.